Starting means for cold electrode vacuum tube devices



June 21, 1960 SKELLETT ETAL 2,942,146

STARTING MEANS FOR COLD ELECTRODE VACUUM TUBE DEVICES INVENTORS asp/van:G- F/A=7'H 44 5K";- M 8x544577- BY 67/;I WW @048 ATTORNEYS United ASTARTING MEANS FOR COLD ELECTRODE VACUUM TUBE DEVICES Albert M.Skellett, Madison, andBernard G. Firth,

Newark, N.J., assignors to Tong-Sol Electric corporation of DelawareFiled Nov. 21, 1957, Ser. No. 697,928

- 9 Claims. (Cl. 315-512) "The present invention relates to coldelectrode vacuum tube devices of the type wherein the current carriersare electronsemitted from an oxide coating on one electrode andcollected by one or more electrodes maintainedat a positive potentialwith respect to the coated electrode. In such type of device the oxidecoating, which may be magnesium oxide, is relatively porous and for mosteffioient emission of electrons comprises a' uniform distripractical forcommercial devices or require too long-a 'timeto bring the tube to fulloperating current conditions. Thepresentinvention comprises novel meansfor starting conduction in cold cathode vacuum tubes that "does not havethe disadvantages inherent in starting means heretofore employed.

V The novel starting means of the invention operates by "field emissionand comprises an additional electrode provided with a fine pointdirected toward the coated electrode'jor cathode and maintained duringstarting at a relatively high negative potential with respect to thecoated electrode. be located between thecoated cathode and the collectorelectrode or on the side of the collector electrode remote from thecathode.

" .For abetter understanding of the invention and of specificembodiments thereof, reference. may be had to thefaccompanying drawingof which:

V Fig. l is a perspective view, partly cut away, of a cold cathodevacuum tube provided with. a starting electrode embodying the invention;V L Fig. '2 is a diagrammatic horizontal section view'taken on theline22 of Fig. 1; p

i Fig. 3 is a diagrammatic partial vertical sectional view through atube like that of Fig. 1 but illustrating a differentlocation within theenvelope of the starting electrode;

. 'QFi'gLA-Yis a horizontalsectional view taken on the line of Fig.3;

Fig.1 5, 'is a diagrammatic horizontal sectional view similar to Figs. 2and 4 but showing" two. starting electrodes; and' Fig. 6 isa view of thepoint of-the.starter-. electrode on a greatlymagnified scale.

The cold cathode vacuum tube illustrated in Figs. 1 and '2compri ses anouter envelope. 20f glass sealed 'to a button type glass base 4. Mountedon the base 4 as by means of support rods 6 is a cylindrical metalshield 8 which encompasses a collector grid or anode 10 of wire meshwhich in turn surrounds the'cylindrica'l cold cathode 12. The cathode 12is preferably of nickel and carries a coating 14 of magnesium oxidewhich is porous are Inc., a

ice

. 2 and, as heretofore indicated, in sponge-like form of microscopicstructures. The coating may be applied by spraying, by vacuumevaporation, or by the magnesium burning technique described in US.Patent to D-obiscek et al. No. 2,802,127. The particular method offorming a coating having the proper physical structure and thickness (ofthe order of 28 to 48 microns) forms no part of the present invention.The shield 8, collector electrode 10 and cathode 12 are supportedbetween spaced mica wafers 16. Support leads 18 and 20 for therespective electrodes 10 and 12 are sealed through the base 4- andconnected to pin terminals 22 and 24- respectively. One support post 6for the shield 8 is also sealed through the base and connected to a pinterminal 25. The 'shield- 8 is provided with an opening 26 substantiallymid way of its length and through this opening extends the pointed 'endof a wire electrode 28, the point 29 of the electrodebeing directedtoward the cathode 12. The electrode 28 is supported by a rod which isfused through the base 4 and connected with a fourth pin terminal 32.The electrode 28 may be made of any conductive, refractory andpreferably rather oxidation resistant material of fairly high tensilestrength such as tungsten, stainless steel, iridium, etc. The point 29thereof 'is etched down to a radius of curvature of about half v amicron 'or less. The point is so oriented with respect The tip ofthepointed electrode may to the openings of grid electrode 10 that thereis an inn obstructed path from the point to the cathode coating fortravel of electrons given off by field emission from the point when ahigh negative potential with respect tothe cathode is impressed uponterminal 32. A fine tip or point on tungsten wire may be formed quicklyand 'efiectively by an anodic process in potassium hydroxide solution(0.5 gram-potassium hydroxide for each cc. of water) with one or twocrystals o-fcupric nitrate and a copper cathode under carefullycontrolled conditions of current and voltage. For example, the tip of a.005" diameter tungsten Wire is immersed M to A2" in such solution andthe applied voltagekept constant at precisely 1 volt. The current willfall from about '10 milliamperes to- 2 /2 milliamperes and then start to-suddenly drop ofl. At the start of this second current drop the circuitis quickly opened to prevent etching off the formed point. A 'finetungsten point is thus formed which after washing carefully and dryingis ready to be mounted in the tube.

In Fig. 1 a circuit for starting and operating a tube of the abovedescribed construction is diagrammatically indicated as comprising asource 340i direct current voltage, the negative terminal of which isconnected to pin terminal 24 and the positive terminal of which isconnected. through an adjustable resistor 36 and half of switch 38toterminal '22 and a second source at of direct current potential thepositive terminal of which is connected'to terminal 24 and the negativeterminal of which is connected through a resistor 42 and the other halfof .switch 38 to terminal 32. Source 34. may be of the order of 400voltsand source 40 of 1000 'volts'or more. Resistor 42 is of largeresistance, of the order of 1000 megohms, whereas resistor 36 is ofrelatively low resistance of the order of 5 to 50 kilohms. The circuitconnection for the shield 8 is to the cathode 12 asthe function of theshield is solely toprotect the cathode from glass wall charges of theenvelope resulting from electron bombardment. Such charging of the.glass walls renders the tube unstable and electrically noisy.

With the, above described construction of cold cathode vacuum tube andacircuit of the above type, when switch 38 is closed starting can; beelfectcd iapidly at a current of less than .01 microarnpere throughresistor 42. The starting time depends upon the voltage applied PatentedJune 21 1960 one-half second for an applied voltage of about 1200 voltsto one or two seconds for an applied voltage of 800 volts. Oncestarted,the tube will pass a current. in the order of milliamperes perfsq. cm.of cathode surface between the cathode 12 and electrode which can be'cut01f and re-initiated merely by opening and closing the switch 38.The'high potential gradient at the point 29 produced by the relativelyhigh potential between it'and the other electrodesis such that electronsare given off by field emission frornthe point and'these electronsstrike the magnesium oxide coating causing 7 emission of secondaryelectrons which in turn are drawn to the grid electrode, thus initiatingtheself-sustaining electron emission from the cathode. Thecathode remains cold but glowsv a bluish color'during conduction er electrodecomprising a metallic member having a fine pointed tip directed towardthe cathode and adapted to emit primary electrons by field emission whena high negative potential with respect to the cathode is impressedthereon.

to permit passage of primary electrons therethrough.

3. A cold cathode vacuum tube according to claim 2.

1 including a metallic shield within said envelope having in the tube.Once the tube is started the voltage may be removed 'from'the startingelectrode or left applied as desired.

Starting of a tube such as that described'may be simi- 1 larly effectedat a lower negative starting potential when reference'num'erals as areapplied in Figs. 1 and 2. In:

the construction illustrated in Figs. 3. and 4 the conductive support 30for the starting electrode 28 is positioned between the shield 8 andgrid electrode 10 and the elec-, trode 28 extends through an opening inthe grid 10 so as to bring the'pointed tip of the electrode between thegrid and the coated cathode 12. Whenv the starting elecelectrodesurrounds said cathode and has openings there in, said starter electrodeextending through oneof said trode is positioned as shown in Figs. 3 and4, a small I dark spot may or may notappear on the luminous cathode at alocation aligned with the point of the starting electrode. This'darkspot may be due to an, electrostatic field created by the presence ofthe points or it may be due toslight, contamination'of the coating bythe point during tube processing. So long as the spot remains relativelysmall it does not seriously interfere with the operation of the tube.With this arrangement it has been observed that a negative potential of600 volts with respect to the cathode vw'll start conduction in the sametime as the negative potential of 1000 volts applied to the starter whenpositioned as in Figs. 1 and 2.

Even more rapid starting may be effected when one or more additionalpoint starting electrodes. are provided directed toward different areasof the cathode coating. Fig. 5 illustrates such an arrangement wherepointed starting electrodes 28a and 28b are directed towarddiametrically disposed areas of the cathode coating. Electrodes 28a and28b should be mounted on separate conductive supports and connectedthrough separate resistors and the switch to one common E.M.F. supply.

The invention has now been described with reference to variousembodiments thereof. Obviously various changes in the particularconstruction of tubes could be made without departing from the spirit ofthe invention or the scope of the accompanying claims. For example,although cylindrical electrodes have been described and illustrated,electrodes of other cross sectional shape, such as semi-cylindrical,oval orflat could be employed. Similarly, the protectiveshield could beomitted if desired (though this is not recommended) and variouslocations of the starting electrode, in addition to those suggested,will occur to those skilled in the art.

The following is claimed:

1. A cold cathode vacuum tube comprising an evacuated envelope, acathode therein comprising a metallic member having a surface coatingthereon of the type adapted to emit a copious self-sustaining stream ofelectrons once electron emission is initiated, a collector electrode forelectrons emitted from the cathode and a startan aperture thereinalignedwith said one opening in said collector electrode, said collectorelectrode being positionedv between said'cathode and said shield andsaid starter electrode extending throughsaid aperture.

4. A, cold cathode vacuum tube according to claim'l wherein saidcollector electrode has openings therein and said'starter electrodeextends through one of said openings. 1

5.. A cold cathode vacuum tube according to claim 1 wherein the radiusof curvature of the pointed tip of said starter, electrode is of, theorder of one-half micron.

1 6. The cold cathode vacuum tube accordingto claim 1 l including asecond starter electrode having a fine pointed tip I directedv towardanother portion of said coated electrode and adapted, when avoltage'negative I with respect to-said coated electrode is impressedtherea on to emit primary electrons by field emission.

' .7. The cold cathode vacuum tube according to claim 1 wherein .saidcathode is generally cylindrical and,

coated over the external surface thereof, said collector openings. e

. '8. The cold cathodetube according to claim, 1 where in said cathodeis generallycylindrical andcoated over the external surface thereof,said collector electrode surrounds said cathode and has openingstherein, the tip of said starter electrode being aligned with one of theopenings in said collector electrode.

9. A cold cathode vacuum tube and energizing means therefor comprising avitreous envelope and a base hermetically sealed thereto, a metallicsheet-like electrode within said envelope and mounted on said base, saidelectrode having a magnesium oxide porous coating thereon composed ofminute sponge-like structures, an openwork collector electrode withinsaid envelope and mounted on said 'base in spaced relation to saidsheet-like electrode, a starting electrode within said envelopecomprising a wire of refractory conductive material having a finepointed tip at one end thereof, conductive means for supporting saidstarting electrode from said base with. the pointed tip thereof directedtoward the coating on said sheet-like electrode, means for impressing apotential difference across said sheet-like electrode and said collectorelectrode with the collector electrode at a positive potential withrespectto'saidsheet-like electrode and means for impressing a higherpotential difference across said sheet-like electrode and said starterelectrode with said sheet-like electrode at a higher potential than saidstarting electrode whereby primaryelectrons emitted from the pointed tipof said starting electrode by field emission strike said coating andcause self-sustaining emission of electrons from said coating which aredrawn to said collector electrode.

References Cited in the file of this patent UNITED STATES PATENTS

